Gas chromatography is essentially a physical method of separation in which constituents of a vapor sample in a carrier gas are adsorbed or absorbed and then desorbed by a stationary phase material in a column. A pulse of the sample is introduced into a steady flow of carrier gas, which carries the sample into a chromatographic column. The inside of the column is lined with a liquid, and interactions between this liquid and the various components of the sample—which differ based upon differences among partition coefficients of the elements—cause the sample to be separated into the respective elements. At the end of the column, the individual components are more or less separated in time. Detection of the gas provides a time-scaled pattern, typically called a chromatogram, that, by calibration or comparison with known samples, indicates the constituents, and the specific concentrations thereof, which are present in the test sample. An example of the process by which this occurs is described in U.S. Pat. No. 5,545,252 to Hinshaw.
Various types of sampling devices can be used to obtain a quantity of the analytes from the sample vessels used to collect the samples to be tested and transfer the analytes to the gas chromatograph for the above-described analysis. One common device is a thermal desorption unit, which is often employed to determine the constituents of a particular environment. For example, it is often desired to detect the amount of volatile organic compounds (VOCs) present in a certain sample of air. One way of doing this is by first transporting a tube packed with an adsorbent material into the environment to be tested, and allowing the VOCs in the air to migrate into the tube through natural diffusion, typically termed “diffusive” or “passive sampling.” Alternatively, the VOCs may be collected by drawing a sample of gas (typically ambient air) through such a tube using a small vacuum pump, commonly referred to as “pumped sampling.” In each case, the analytes to be measured (i.e., the VOCs) are retained by and concentrated on the adsorbent as the air passes through the tube.
Once the VOCs are collected in this fashion, the tube is then transported to a thermal desorption unit, where the tube is placed in the flow path of an inert gas, such as Helium or Nitrogen. The tube is subsequently heated, thereby desorbing the analytes, and the carrier gas sweeps the VOCs out of the tube. In some cases, a “trap” is located downstream of the sample tube in order to further pre-concentrate the analytes, and occasionally, remove moisture therefrom, prior to introducing the sample into the chromatographic column. One example is an adsorbent trap, usually cooled to a sub-ambient temperature, which is simply another sorbent tube packed with a suitable adsorbent material, which adsorbs the analytes as the sample gas first passes through the tube, and from which the analytes are then desorbed into the chromatographic column, usually by heating, for subsequent separation and analysis.
Another common sampling device is a headspace sampler. In conventional headspace sampling, sample material is sealed in a vial and subjected to constant temperature conditions for a specified time. Analyte concentrations in the vial gas phase should reach equilibrium with the liquid and/or solid phases during this thermostatting time. The vial is subsequently pressurized with carrier gas to a level greater than the “natural” internal pressure resulting from thermostatting and equilibration. Then the pressurized vial is connected to the chromatographic column in such a way as to allow for the transfer of a portion of the vial gas phase into the column for a short period of time. An example of such a sampling device is disclosed in U.S. Pat. No. 4,484,483 to Riegger et. al. An example of a chromatographic system employing a headspace sampler is disclosed in U.S. Pat. No. 5,711,786 to Hinshaw.
In some applications, the column is directly coupled to a sorbent tube in the sampling device or the device is connected to the column via a transfer line, such as, for example, via a length of fused silica tubing. Other recent applications employ an interface device for performing some additional control or trapping in addition to that already provided by the sampling device, including the thermal desorption system disclosed in U.S. patent application Ser. No. 11/169,935 to Tipler et al., as well as the headspace sampling system disclosed in U.S. Pat. No. 6,652,625 to Tipler, each of which is assigned to the assignee of the present application, and the contents of each of which are herein incorporated by reference in their entirety.
In some embodiments, however, as the column is heated, the viscosity of the gas flowing through it likewise increases. As a result, under isobaric conditions—where the carrier gas is applied at a constant pressure, the flow rate through the column will decrease. Though this has no detrimental effect on system performance in some applications, in other applications that employ a flow-sensitive detector, such as a mass spectrometer, the effect on performance can be dramatic.
Some gas chromatographs are equipped with programmable pneumatic controls, and thus, the chromatograph is able to compensate for such changes in gas viscosity by increasing the column inlet pressure at a rate calculated to maintain a constant flow rate through the column, which requires constant knowledge of the column temperature in order to calculate the viscosity at that temperate and make the appropriate adjustments to the applied pressure. However, this solution is not available when the gas pressure is controlled on a device remote from the chromatograph, such as on a thermal desorption unit or a headspace sampler, where the gas is supplied from the device along a transfer line and the remote device does not know the temperature of the column.